1. Field of the Invention
The present invention relates to a ceramic material and a resistive element including the ceramic material.
2. Description of the Related Art
Modules and motors handling large currents are recently used in electric vehicles, hybrid vehicles, etc., which are becoming increasingly popular. In these modules etc., when an inrush current occurs at power-on (or at a start of a motor) and the inrush current excessively flows through the modules etc., electronic components and ICs therein may be damaged or destroyed, and this must therefore be dealt with. For example, an inrush current generated at a start of a motor of an electric vehicle may reach several hundred amperes, so that it is required to sufficiently suppress the inrush current. A use of a thermistor element is being studied as a countermeasure against such an inrush current.
An NTC (negative temperature coefficient) thermistor is conventionally known as an inrush-current suppressing thermistor element. The NTC thermistor for suppressing an inrush current is generally an element having a room-temperature resistance of a little less than 10Ω and made of an NTC thermistor material having a room-temperature specific resistance of about several hundreds to several thousands of Ω·cm. However, such NTC thermistors with a small specific resistance do not have a sufficiently large resistance change (that may be evaluated by a B-constant) between a low temperature state and a high temperature state and have drawbacks, such as a relatively large power loss due to a residual resistance while a steady current flows (ON state, high-temperature state). On the other hand, those with a large specific resistance have a large resistance change (B-constant) between a low temperature state and a high temperature state. However, an element size problematically must increase in order to reduce the element resistance. This is because a positive correlation generally exists between a specific resistance of a conductive material and the B-constant, and when the specific resistance is reduced, the B-constant becomes smaller, so that a low specific resistance and a high B-constant are difficult to achieve at the same time. This problem becomes more apparent in applications requiring elements with lower resistance, and conventionally known NTC thermistor materials result in an extremely large element size and are difficult to use due to problems of mounting, for example.
Therefore, studies have been conducted for using a CTR (critical temperature resistor) as the inrush-current suppressing thermistor. The CTR has characteristics (hereinafter simply referred to as “CTR characteristics”) exhibiting a steep resistance decrease at a certain temperature or in a temperature range (transitioning from an insulator to a metal state) when the temperature is raised, and has an extremely large B-constant as compared to NTC thermistors, in which the resistance gradually decreases as a temperature rises.
A ceramic material proposed as a ceramic material having the CTR characteristics has a structure represented by a chemical formula R11-xR2xBaMn2O6, and is characterized as follows:
(1) when R1 is composed of Nd and R2 is composed of at least one of Sm, Eu, and Gd, x satisfies 0.05≤x≤1.0;
(2) when R1 is composed of Nd and R2 is composed of at least one of Tb, Dy, Ho, Er, and Y, x satisfies 0.05≤x≤0.8;
(3) when R1 is composed of at least one of Sm, Eu, and Gd and R2 is composed of at least one of Tb, Dy, Ho, and Y, x satisfies 0≥x≤0.4; and
(4) when R1 is composed of at least one of Sm, Eu, and Gd and R2 is composed of at least one selected from the remainder not selected as R1 out of Sm, Eu, and Gd, x satisfies 0≤x≤1.0 (see WO 2012/056797).
The ceramic material described in WO 2012/056797 is an A-site-ordered Mn compound in which a rare-earth element and barium entering the A site of the perovskite structure are ordered, and exhibits the CTR characteristics. It is described in WO 2012/056797 that this ceramic material exhibits a steep resistance change at around 100° C. as shown in FIG. 2 of WO 2012/056797, for example, and is suitable for an inrush-current suppressing thermistor element.
An inrush-current suppressing thermistor element, or particularly, a thermistor element for high-power application, desirably has a low room-temperature specific resistance as compared to an inrush-current suppressing element that is made using a conventional NTC thermistor material. If the room-temperature specific resistance of the ceramic material used to make the inrush-current suppressing thermistor element is too high, an element is increased in size (increased in area and made thinner) so as to achieve a required resistance level for the element, causing a major problem in practical use due to a reduction in mechanical strength and an increase in mounting area. For functioning as an inrush-current countermeasure element, self-heating must occur due to the inrush current so that the element reaches the temperature of the steady state and enters the ON state (achieves a low resistance). However, since the large element size leads to a large heat capacity as well as a large heat dissipation area, the responsiveness to the inrush current decreases, or an insufficient rise in temperature results in a high resistance in the on-state and a large power consumption, which is not acceptable.
Furthermore, to effectively suppress the inrush current over a relatively wide temperature range from a low temperature to the transition temperature and minimize the power consumption by the thermistor element while the steady current flows, it is desirable that the inrush-current suppressing thermistor element exhibits a steep resistance change (i.e., large B-constant) due to a temperature rise and that the temperature (transition temperature) causing the element to exhibit this steep resistance change is within a range of 80° C. to 180° C.
As a result of studies by the inventor of the present invention, it was discovered that although the ceramic material described in WO 2012/056797 has a room-temperature specific resistance at an acceptably low level for the inrush-current suppressing thermistor element and exhibits a steep resistance change (decrease) due to a temperature rise, the resistance is increased by a heat cycle test.
If a thermistor element is used for suppressing an inrush power, the element has a lower resistance because of an increase in temperature due to self-heating at the time of power-on when the inrush current occurs, and has a higher resistance because of a decrease in temperature at the time of power-off, so that a history of temperature transition between a low temperature state and a high temperature state is repeated in practical use. Therefore, the increased resistance value revealed by the heat cycle test may also be generated in actual use, which may cause a malfunction of a module.
Therefore, the ceramic material described in WO 2012/056797 is inferior in terms of reliability (heat cycle resistance) and is not necessarily satisfactory as a material for the inrush-current suppressing thermistor element.